1. Field of the Invention
The present invention relates generally to a Visible Light Communication (VLC) system which supports unicast, multicast, and broadcast communication modes, and more particularly, to a method and apparatus for outputting a visibility frame for a communication link connection during a communication link release in a VLC system.
2. Description of the Related Art
With the improvement of luminous efficiency and competitive prices, Light Emitting Diodes (LEDs) have been widely used not only in special lighting markets for mobile devices, displays, vehicles, traffic lights, advertising boards, etc., but also in general lighting markets for fluorescent lamps, incandescent lamps, etc. Further, interest has increased in an optical wireless technology, which is complementary to a Radio Frequency (RF) technology, due to the exhaustion of RF band frequencies, the possibility of interference between various wireless communication techniques, the increased demand for communication security, the advent of ubiquitous super-high speed communication environments in a 4th Generation (4G) wireless technology, and the like. Accordingly, research on VLC using LEDs is being conducted.
FIG. 1 illustrates a general VLC system. The general VLC system includes a light source 10 and a VLC terminal 20. The light source 10, which includes LEDs or Laser Diodes (LDs), serves as an illumination and transmits and receives data using visible light. The VLC terminal 20 is equipped with a visible light transmission/reception module, and transmits and receives data to and from the light source 10. The VLC terminal 20 may be a mobile terminal, such as a cellular phone or a Personal Digital Assistant (PDA), or may be a fixed terminal of a desktop type. The VLC system may be more effectively used when combined with other systems using a wired or wireless communication medium.
Generally, a plurality of light sources 10 may be installed for VLC. When a plurality of light sources are installed at regular intervals, substantial service areas provided by the light sources may appear as three possible cases as shown in FIGS. 2A to 2C. FIGS. 2A to 2C illustrate service areas provided by two light sources. Service areas provided by light sources 31 and 33 may overlap each other as shown in FIG. 2A, may contact each other at their boundaries as shown in FIG. 2B, or may be separated from each other so that a non-service area is present as shown in FIG. 2C.
When providing a unicast, multicast or broadcast communication service to multiple user terminals using VLC, the multiple user terminals may be located in a service area provided by one light source and each terminal may receive the same or individual data. In VLC, the transmission and reception of data can be smoothly performed when communication links of a transmitting side and a receiving side are aligned in both directions due to a characteristic of a transmission medium. Even in one-way communication, the communication links should be formed so as to accurately direct a visible light signal generated from a visible light transmitter of the transmitting side to a visible light receiver of the receiving side. However, terminals that receive data from a light source may encounter disconnected communication links or an improperly transmitted visible light signal, due to external environment factors, movement of the terminals, obstacles, etc. Moreover, when there are a plurality of terminals that receive data from one light source, a communication link connection may be released from one of the plurality of terminals.